Method to reduce flue gas NOx

ABSTRACT

A method of decreasing the concentration of nitrogen oxides in a combustion flue gas in which a nitrogen reducing agent is introduced together with the overfire air to mixes with the products of primary combustion along with the overfire air. The nitrogen agent reduced NOx as it passes through the temperature regime that is optimum for the NOx reduction as overfire air and flue gas mix. The transition from low to high temperature effectively eliminates ammonia slip. Additionally, the nitrogen agent may be mixed with the overfire air stream in such a manner that it is optimally shielded from early mixing with the products of primary combustion, where a portion of the overfire air reacts initially with any residual carbon monoxide (CO) that would otherwise interfere with the NOx reduction chemistry.

BACKGROUND OF THE INVENTION

This invention relates to nitrogen oxide (NOx) emission controls forcombustion systems such as boilers, furnaces, incinerators and otherlarge combustion systems (collectively referred to herein as “boilers”).In particular, the invention relates to reduction of NOx emissions byselective reduction of nitrogen oxides to molecular nitrogen.

Emissions of smoke from boilers are eliminated or at least greatlyreduced by the use of overfire air (OFA) technology. OFA stages thecombustion air such that most of the air flows into a primary combustionchamber of the boiler and a portion of the combustion air is diverted toa burnout zone, downstream of the flame. OFA air facilitates combustionof smoke particles and smoke particle precursors.

Other types of air pollutants produced by combustion include oxides ofnitrogen, mainly NO and NO₂. Nitrogen oxides (NOx) are the subject ofgrowing concern because of their toxicity and their role as precursorsin acid rain and photochemical smog processes. There is a long felt needfor cost-effective techniques to reduce NOx emissions generated byboilers.

A conventional NOx reduction technique is Selective Non-CatalyticReduction (SNCR) that injects a nitrogen agent into the flue gas underconditions that cause a noncatalytic reaction to selectively reduce NOxto molecular nitrogen. The NOx reduction is selective because much ofthe molecular oxygen in the flue gas is not reduced. In SNCR, a nitrogenbearing reagent, e.g. HN₃, urea, or an amine compound, is injected intothe flue gas stream at a temperature optimal for the reaction of NH₂ andNH radicals with NO reducing it to molecular nitrogen. The optimumtemperature for such reactions is centered at approximately 1800° F. Atsubstantially higher temperatures, the reagent can be oxidized to NO. Atsubstantially lower temperatures, the reagent may pass through the fluegases unreacted, resulting in ammonia slip. An optimal range oftemperatures to reduce NOx using SNCR methods is narrow and generallyabout 1600° F. to about 2000° F., wherein “about” refers to atemperature difference of plus or minus 25 degrees.

Flue gases reach temperatures well above 2000° F., but cool as they flowthrough the boiler. To allow the flue gases to cool before the nitrogenagent is released, schemes have been developed to inject relativelylarge droplets or particles of the agent into the flue gas, such as withthe overfire air. The large droplets and particles are sized so as torelease the nitrogen agent after the flue gas has cooled. See U.S. Pat.No. 6,280,695. The large droplets delay the release of the reagent inthe flue gas stream until the bulk temperature of the flue gas cools toa temperature window of about 1600° F. to 2000° F.

BRIEF DESCRIPTION OF THE INVENTION

The present invention, in one embodiment, provides for a process forremoving nitrogen oxides by injecting reducing agent into a gas streamwhile simultaneously minimizing ammonia slip. In a first embodiment, theinvention is a method of decreasing the concentration of nitrogen oxidesin a combustion flue gas including the steps of: forming a combustionflue gas in a combustion zone; providing overfire air and droplets of asolution or a gas of a selective reducing agent in a burnout zone, thedroplets having a small average size to promote fast reduction of thenitrogen oxides; mixing the overfire air and the selective reducingagent with the combustion flue gas in the burnout zone at a temperatureabove an optimal temperature range for reduction of the nitrogen oxidesin the flue gas; as the combustion flue gas heat the overfire air andthe selective reducing agent to the optimal temperature range, reducingthe nitrogen oxides with the reducing agent, and continuing to increasethe temperature of the overfire air and the selective reducing agentbeyond the optimal temperature range with the flue gas.

The invention may also be embodied as a method of decreasing theconcentration of nitrogen oxides in a combustion flue gas, comprising:forming a combustion flue gas in a combustion zone, the combustion fluegas comprising nitrogen oxides; providing overfire air and droplets ofan aqueous solution or gas of a selective reducing agent in a burnoutzone, the droplets or gas having an initial average size of less than 50microns, and contacting the combustion flue gas with the overfire airand the selective reducing agent in the burnout zone to decrease theconcentration of nitrogen oxides therein.

The invention may further be embodied as a combustion apparatus forcombusting comprising: a boiler defining an enclosed flue gas pathhaving a combustion zone and a burnout zone, wherein flue gas is formedin the combustion zone and the combustion flue gas comprising nitrogenoxides; a fuel injector aligned with an introducing fuel into thecombustion zone and a combustion air injector aligned with andintroducing air into the combustion zone; an overfire air systemadjacent the burnout zone comprising an overfire air port adjacent theburnout zone and through which overfire air flows into the burnout zone;a nitrogen reagent injector having an outlet aligned with the overfireair system and injecting nitrogen reagent gas or small droplets intosaid overfire air, wherein said small droplets have an average diameterof no greater than 50 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a combustion system having annitrogen agent injector in an overfire air port.

FIG. 2 is a graph of combustion flue gas temperature vs. time.

FIGS. 3 to 6 are charts showing the effect on NOx emission levels due toinjection of a gaseous nitrogen agent (NH₃) at different stoichiometricratios and different flue gas temperatures.

FIG. 7 is a table of computer generated predictions of NOx emissionreductions for various nitrogen agent droplet sizes, flue gastemperatures and other boiler operating parameters.

FIG. 8 is a chart of computer generated predictions of NOx emissionreductions for various nitrogen agent droplet sizes and flue gastemperatures.

DETAILED DESCRIPTION OF THE INVENTION

A gaseous or small droplet (less than 50 micron in diameter) nitrogenbearing reagent (a “nitrogen agent”) is introduced with stagedcombustion air, e.g., OFA air, downstream of the primary combustion zoneto reduce NOx in the flue gas of a boiler. In staged combustion, aportion of the air required to complete combustion (overfire air) isinjected downstream of the primary combustion process, such as where theflue gas products of primary combustion have cooled to approximately2400° F. to 2600° F. The nitrogen agent in gaseous form or smalldroplets (<50 about microns) in an aqueous solution is introducedtogether with the staged air into the extremely hot flue gas. The stagedcombustion air and nitrogen agent are rapidly heated to the flue gastemperature of above 2000° F.

A surprising discovery has been made that NOx reduction can be achievedwith SNCR by a release of a gaseous or small droplet nitrogen agent intoan area at or near the OFA injector, where the bulk flue gas temperatureis too hot for optimal SNCR. A nitrogen agent of gaseous ammonia orsmall droplets, e.g., average diameter of less than about 50 microns, isinjected into the OFA system prior to or simultaneous with the mixing ofOFA air with the NOx containing flue gas. The nitrogen agent gas orsmall droplets provides a quick nitrogen reagent release, such as in aperiod less than about 0.1 to 0.3 second. The nitrogen release occurs asthe relatively cool nitrogen agent and overfire air mixture is heated byflue gases through an optimal temperature window for SNCR and to hottertemperatures. The nitrogen may be within the optimal temperature windowfor a brief period, e.g., 0.1 to 0.3 seconds. By introducing thenitrogen agent as a gas or small droplets, the agent is quickly releasedduring the brief period of the optimal temperature window. The quickrelease ensures that the nitrogen agent contacts NOx in the flue gaswhen the agent is within the optimal temperature window. In addition,the release occurs close to the overfire air injector outlet, wherevigorous mixing occurs between overfire air jet and flue gas streams.

Preferably, the droplet size of the agent is adjusted so that theaverage droplet lifetime in the flue gas and OFA is approximately equalto the period of the optimal temperature window and/or the period duringwhich the overfire air mixes with the combustion flue gas. In general, asuitable initial average size of the droplets injected into the overfireair is less than about 50 microns in diameter. The preferred dropletsize is a size of the droplets as injected into the overfire air, e.g.,the size of the droplets before evaporation. Preferably, the averagedroplet lifetime is less than about one-tenth (0.1) to 0.3 of a second.

FIG. 1 is a schematic representation of a combustion system 10 such asused in a coal-fired boiler and adapted for the methods of the presentinvention. The combustion system 10 includes a combustion zone 12, areburn zone 14 and a burnout zone 16. The combustion zone 12 is equippedwith at least one, and preferably a plurality, of main burners 18 whichare supplied with a main fuel such as coal through a fuel input 20 andwith air through an air input 22. The main fuel is burned in combustionzone 12 to form a combustion flue gas 24 that flows from combustion zonetoward burnout zone 16, in a “downstream direction”.

If there is no reburn zone, then all of the heat source, e.g., coal, isinjected into the combustion zone through the main burners 18. When areburn zone 14 is included in the combustion system, typically about 70%to 95% of the total heat input is supplied through main burners 18 andthe remaining 5% to 30% of heat is supplied by injecting a reburn fuel,such as natural gas, coal or oil through a reburn fuel input 26.Downstream of combustion and reburn zones, overfire air (OFA) 28 isinjected through an overfire air input 30 into the OFA burnout zone 16.Downstream of the burnout zone, the combustion flue gas 24 passesthrough a series of heat exchangers 32 and a particulate control device(not shown), such as an electrostatic precipitator (ESP) or baghouse,that removes solid particles from the flue gas, such as fly ash.

Combustion flue gas 24 is formed by burning conventional fuels, e.g.,coal, in any of a variety of conventional combustion systems. Theburnout zone 16 is formed by injecting overfire air 28 in a region ofthe combustion system downstream, i.e., in the direction of combustionflue gas flow, from the combustion zone. Combustion devices that includea combustion zone for oxidizing a combustible fuel and a burnout zonecan be adapted to receive a mixture of a nitrogen reducing agent andoverfire air as a means to reduce NOx emissions. For example, thecombustion and burnout zones may be provided in a power plant, boiler,furnace, magnetohydrodynamic (MHD) combustor, incinerator, engine, orother combustion device.

A selective reducing agent (nitrogen agent or N-agent) 34 is injectedinto to the overfire air prior to or concurrently with injection of theoverfire air 28 into the burnout zone 16. The nitrogen agent may beinjected through a center nozzle 36 or other injection system into thecenter of the OFA air flow, e.g., OFA inlet port 30 connection to theflue gas stream. The OFA port 30 may comprise inlet ports at the cornersof the boiler and where the ports are at the same elevation in theboiler. The nozzle 36 may inject that nitrogen agent at the inlet of theOFA to the flue gas stream or substantially upstream of the inlet andwell before the OFA mixes with the flue gas. Aqueous solutions of theselective reducing agent can be injected into the OFA air usingconventional injection systems commonly used to generate small droplets.The nitrogen-agents can be injected by gas-liquid injectors such asvarious atomizers. Suitable atomizers include dual-fluid atomizers thatuse air or steam as the atomizing medium, as well as suitably designedpressure atomizers.

The nitrogen-agent injection system 36 may be capable of providingdroplets with an average size that can be adjusted. The initial averagesize distribution of the spray droplets may be substantiallymonodisperse, e.g., having fewer than about 10% of the droplets withdroplet sizes (i.e., diameter) less than about half the average dropletsize, and fewer than about 10% of the droplets having a droplet size ofgreater than about 1.5 times the average droplet size. The average sizeof the droplets injected into the OFA can be determined, by selectingdroplet sizes that optimize the droplet evaporation time from thenitrogen agent.

The terms nitrogen oxides and NOx are used interchangeably to refer tothe chemical species nitric oxide (NO) and nitrogen dioxide (NO₂). Otheroxides of nitrogen are known, such as N₂O, N₂O₃, N₂O₄ and N₂O₅, butthese species are not emitted in significant quantities from stationarycombustion sources (except N₂O in some combustion systems). The termnitrogen oxides (NOx) is used generally to encompass all binary N—Ocompounds, However, NOx also particularly refers to the NO and NO₂.

The terms selective reducing agent and nitrogen agent are usedinterchangeably to refer to any of a variety of chemical species capableof selectively reducing NOx in the presence of oxygen in a combustionsystem. In general, suitable selective reducing agents include urea,ammonia, cyanuric acid, hydrazine, thanolamine, biuret, triuret,ammelide, ammonium salts of organic acids, ammonium salts of inorganicacids, and the like. Specific examples of ammonium salt reducing agentsinclude, ammonium sulfate, ammonium bisulfate, ammonium bisulfite,ammonium formate, ammonium carbonate, ammonium bicarbonate, and thelike. Mixtures of these selective reducing agents can also be used. Theselective reducing agent can be provided in a solution, preferably anaqueous solution or as a gas. One preferred selective reducing agent isurea in aqueous solution.

Locating a gaseous or small droplet SNCR injection system with theoverfire injection system is generally convenient and avoids the crampedand hot space at the primary combustion zone of a boiler or furnace.Using overfire air to introduce a nitrogen agent allows the gas and/ordroplets of the agent solution to be injected into an NOx containingflue gas and then quickly heated to a temperature appropriate for SNCRNOx reduction without the expense and downtime of installing aninjection system in a high temperature region of the boiler, furnace orother combustion system.

The stoichiometric ratio of the amount of selective reducing agent inthe overfire air to the amount of NOx in the flue gas being treated isabout 0.4 to about 10, and preferably about 0.7 to about 3. Thestoichiometric ratio is the ratio of number of moles of nitrogen atomsin the selective reducing agent to number of moles of nitrogen atoms inthe NOx.

The selective reducing agent may be provided in an aqueous solution inoptimized drop form, or in a gas that is injected into the overfire airbefore injection of the overfire air into the reburn zone, concurrentlywith injection of the OFA air into the reburn zone, or both. Thenitrogen-agent solution or gas may also be injected into the OFA of acombustion system without reburning. The nitrogen-agent aqueous solutioncan contain the selective reducing agent in any suitable concentration,such as from about 5% to about 90% by weight. A preferred concentrationrange for urea is about 10% to about 50% by weight.

The nitrogen-agents can be injected with the OFA without previouslyinjecting reburning fuel into the flue gas. Further, the N-agents can beinjected with recirculated flue gas which can serve the same purpose asOFA. For example, recirculated flue gas enriched by oxygen or air can beinjected along with the N-agents through the same or separate injectors.

FIG. 2 is a graph 50 showing the residence time of a nitrogen agent inOFA and flue gas verses gas temperature. As represented by the plot 52of mean OFA temperature for a nitrogen agent having droplets smallerthan 50 microns, the mixture of nitrogen agent and OFA air increasesfrom about 600° F. to 2,500° F. in about 0.4 second as the mixtureenters the flue gas from the OFA port 30. As the overfire air, nitrogenagent and flue gas mix, the nitrogen agent is heated through atemperature window 54 that is optimal for SNCR, e.g., 1,600° F. to2,000° F. This temperature window is brief, e.g., about 0.1 or 0.2seconds.

During the brief temperature widow 54 of optimal SNCR, the gaseous orsmall diameter nitrogen agent reduce substantial amounts of NOx in theflue gas. The reduction of NOx is further promoted if the OFA and fluegas vigorously mix at the same time that the nitrogen agent is heatedthrough the optimal temperature window. Vigorous mixing and rapid OFAheating generally occur as the overfire air enters the flue gas from theOFA inlet port 30. Accordingly, the reduction of NOx due to SNCR occurswhere the overfire air enters and mixes with the flue gas. Moreover,injecting the nitrogen agent through a nozzle 36 at the or near the OFAinlet port and/or aligned with the center of the overfire air streamseems to promote the reduction of NOx in the fuel gas. The OFA airshielding the nitrogen agent reacts initially with any residual carbonmonoxide (CO) that would otherwise interfere with the SNCR chemistry.

The nitrogen agent and OFA are quickly heated by the flue gas (see plot55) to a temperature beyond the optimal SNCR window 54. Much of the NOxin the flue gas has already been reduced when the OFA and any remainingnitrogen agent are heated to the 2,500° F. flue gas temperature (seewhere plot 52 of the OFA merges with plot 55 of the flue gas) which istoo hot for optimal SNCR. As the flue gas 55 cools to 2,000° F. andbelow, it flows beyond the nose plane 56 of the combustion system, e.g.,a boiler, and to the super-saturated steam (SSH) unit 58 and steamreheat (RH) unit 60 that receive the flue gas passing out of the boiler.

Tests of NOX performance with gaseous ammonia injection into overfireair were performed in 1.0 MMBTU/hr Boiler Simulator Facility (BSF) whichprovides an accurate sub-scale simulation of the flue gas temperaturesand compositions found in a full-scale boiler. The BSF is described inU.S. Pat. No. 6,280,695. For the tests, the BSF was fired on naturalgas. A specially constructed overfire air injector was placed at aspecific flue gas temperature. The injector consisted of an axialnitrogen carrier tube with an ID of 1.875 inches surrounded by anannular overfire air tube with an inside diameter (ID) of 0.25 inches.Ammonia was added to either the nitrogen carrier or overfire air streamto reduce NOx emissions. The injector was aligned on the BSF centerlineand aimed downward (i.e., co-current with the flue gas).

Primary flame stoichiometric ratio (SR₁) in tests was 1.0 and 1.05.Sufficient OFA was injected to maintain final SR (SR₃) constant at 1.20which corresponds to about 3% excess O₂ in flue gas. These conditionswere defined as baseline. The BSF burner system at baseline conditionsgenerated controlled initial NOx levels of 185 ppm and 205 ppm at 0% O₂at SR₁ equaled 1.0 and 1.05, respectively. SR₁ is stoichiometric air tofuel ratio in the primary combustion zone 12; SR₂ is the same ratio butin the reburn zone 14, and SR₃ is the stoichiometric ratio in theburnout zone 16. OFA was injected at flue gas temperatures of 2450° F.and 2350° F. Gaseous ammonia was injected at a concentration ratio (NSR)of 1.5. The concentration ratio (NSR) is the ratio of moles of atoms ofnitrogen in the ammonia to moles of atoms of nitrogen in NOx.

The Baseline, Carrier NH₃ and OFA NH₃ NOx emission levels shown on thecharts of FIGS. 3 to 6 relate to tests conducted in the BSF undersimilar conditions, except that: the Baseline bar relates to testsconducted without a nitrogen agent; the Carrier NH₃ bar relates toinjection of the nitrogen agent gas (NH₃) along with nitrogen as acarrier down a center-pipe injector that discharges the agent into thecenter of the overfire air as they both enter the flue gas, and the OFANH₃ bar relates to the injection of a nitrogen agent gas in with theoverfire air before they both mix with flue gas.

FIGS. 3 to 6 show the effect of gaseous ammonia injection via the OFAport on NOx emissions. Baseline NOx concentration was lower for SR₁equal to 1.0 than when SR₁ was equal 1.05 as is shown by a comparison ofthe bar chart of test data taken out SR₁=1.0 shown in FIG. 3 with thechart of data taken at SR₁=1.05 shown in FIG. 4. A similar comparison ismade with respect to FIGS. 5 and 6. The test results demonstrate thatinjection of gaseous ammonia (NH₃) into the OFA results in 20% to 45%NOx reduction, depending upon the flue gas temperature at the point ofoverfire air injection and upon the main burner stoichiometry (SR₁).Better performance was achieved at when the overfire air was injected atcooler flue gas temperatures. NH₃ injection through center of the OFAport using nitrogen gas as a carrier provided slightly better NOxreduction than injection of the ammonia (NH₃) into the overfire airstream before the OFA stream mixed with the flue gas.

A computational Fluid Dynamics (CFD) analysis was performed to predictSNCR performance. The CFD model solved the transport equations forcontinuity, momentum, energy and species. The appropriate models wereapplied to solve turbulence, radiation, discrete phase trajectories andcombustion. For the CFD study, a 160 MW tangential-fired boiler wasevaluated with a nitrogen reagent injected into the overfire airinjectors at different operating conditions. Prior to simulating thenitrogen reagent injection process, the CFD model was validated usingbaseline field test data and mean temperature profiles from a in-housethermal model for full load operating conditions. The flue gas flowprofiles into the injector region of the model were based on those fromthe physical flow modeling test results.

The CFD model was run for various operating conditions to investigatethe impact of reagent droplet size, firing rate reflected by the gastemperature immediately below the OFA injector, CO concentration belowthe OFA injector, and the stoichiometric ratio of reagent nitrogen tobaseline NOx nitrogen (NSR) on SNCR NOx trim performance. In this case,NOx trim refers to NOx reduction exceeding that for pure overfire airconditions.

FIGS. 7 and 8 show predicted NOx trim (emission reduction) performancefrom the CFD model at different boiler process conditions. The NOxreduction process is most effective for flue gas temperatures less thanabout 2500° F. at the OFA injection port and at a CO concentration ofzero. For flue gas temperatures (at the OFA port) less than 2500° F. andCO concentrations 200 ppmv and below, NOx trim increases (wherein areduction of NOx emission occurs) as the nitrogen agent droplet sizedecreases, indicating that nitrogen reagent released near the OFAinjectors at the flue gas/OFA mixing zone (e.g., burnout zone 16) iseffective in reducing NOx.

Both pilot-scale experiments and CFD results show that injecting smalldroplets (having an average diameter of less than about 50 to 60microns) into overfire air permits reagent release in the fluegas/overfire air mixing front improving NOX trim relative to dropletsizes greater than about 50 microns.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method of decreasing the concentration of nitrogen oxides in acombustion flue gas comprising: a. forming a combustion flue gas in acombustion zone, the combustion flue gas comprising nitrogen oxides; b.introducing overfire air and the droplets of a solution or particles ofa selective reducing agent in a burnout zone; c. mixing the overfire airand the selective reducing agent with the combustion flue gas in theburnout zone at a flue gas temperature above an optimal temperaturerange for reduction of the nitrogen oxides using the reducing agent,wherein the optional temperature range is above 1600° F.; d. heatingwith the combustion flue gas, the overfire air and the droplets orparticles of the selective reducing agent to the optimal temperaturerange; e. reducing the nitrogen oxides with the selective reducing agentheated to the optimal temperature range, and f. continuing to increasethe temperature of the overfire air and the selective reducing agentbeyond the optimal temperature range with the flue gas.
 2. The method ofclaim 1 wherein the optimal temperature range during step (d) occurs ina brief period of less than 0.3 second and the reduction of the nitrogenoxides occurs during the brief period.
 3. The method of claim 1 whereinthe droplets or particles have an average size of no greater than 60microns.
 4. The method of claim 1 wherein the small average size ofdroplets or particles is no greater than 50 microns.
 5. The method ofclaim 1, wherein the step of mixing the overfire air and the selectivereducing agent with the combustion flue gas occurs as the flue gas is ina temperature range of about 2500° F. to about 2000° F., and the optimaltemperature range is 1600° F. to less than 2000° F.
 6. The method ofclaim 1, wherein the step of providing the overfire air and theselective reducing agent comprises adding the selective reducing agentto the overfire air concurrently with injecting overfire air into thecombustion flue gas in the burnout zone.
 7. The method of claim 1,wherein the step of providing the overfire air and the selectivereducing agent comprises adding the selective reducing agent to theoverfire air prior to injecting the overfire air into the burnout zone.8. The method of claim 1, wherein the selective reducing agent isinjected into a center portion of a stream of overfire air.
 9. Themethod of claim 1, wherein the selective reducing agent is injected intoan upper portion of a stream of overfire air.
 10. The method of claim 1,wherein the selective reducing agent reduces the nitrogen oxides whenthe flue gas is at an average temperature above 2000° F., and themixture of flue gas, overfire air and reducing agent is at an averagetemperature of about 1600° F. to about 2000° F.
 11. The method of claim1, wherein the solution is an aqueous solution.
 12. The method of claim1, wherein the selective reducing agent is provided in a stoichiometricratio of about 0.4 to about 10, wherein the stoichiometric ratio is aratio of moles of atoms of nitrogen in the selective reducing agent tomoles of atoms of nitrogen in the nitrogen oxides.
 13. The method ofclaim 12, wherein the stoichiometric ratio is in a range of 0.7 to 3.14. The method of claim 1, wherein the droplets are formed to have aninitial average size distribution with fewer than about 10% of thedroplets having a droplet size greater than about 1.5 times an averagedroplet size.
 15. The method of claim 1, wherein the mixture of overfireair and droplets of a solution or gas of the selective reducing agent isformed by injecting the droplets into the overfire air.
 16. The methodof claim 1, wherein the concentration of the selective reducing agent inthe solution is about 5% by weight to about 90% by weight.
 17. Themethod of claim 1, wherein the overfire air is injected through at leasttwo ports located at different levels with selective reducing agentinjected through an upper port of said at least two ports.
 18. Themethod of claim 1, wherein then overfire air is a recirculating O₂enriched flue gas.
 19. A method of decreasing the concentration ofnitrogen oxides in a combustion flue gas, comprising: (a) forming acombustion flue gas in a combustion zone, the combustion flue gascomprising nitrogen oxides; (b) providing overfire air and droplets ofan aqueous of a selective reducing agent in a burnout zone, the dropletsor particles having an initial average size of less than 50 microns; (c)introducing the overfire air and the selective reducing agent intocombustion flue gas in the burnout zone at the flue gas temperatureabove 2000 degrees F.; (d) mixing the overfire air and the selectivereducing agent with the combustion flue gas in the burnout zone at aflue gas temperature above an optimal temperature range for reduction ofthe nitrogen oxides using the reducing agent, wherein the optionaltemperature range is above 1600° F.; (e) heating with the combustionflue gas, the overfire air and the droplets or particles of theselective reducing agent to the optimal temperature range; (f)decreasing the concentration of nitrogen oxides in the flue gas byreducing the nitrogen oxides with the selective reducing agent, and (g)continuing to increase the temperature of the overfire air and theselective reducing agent beyond the optimal temperature range with theflue gas.
 20. The method of claim 19, wherein the selective reducingagent is selected from a group consisting of urea, ammonia, ammoniumsalts of organic acids, ammonium salts of inorganic acids, and mixturesthereof.
 21. The method of claim 19, wherein the step of providing theoverfire air and the selective reducing agent comprises adding theselective reducing agent to the overfire air concurrently with injectionof the overfire air into the burnout zone.
 22. The method of claim 19,wherein the step of providing the overfire air and the selectivereducing agent comprises adding the selective reducing agent to theoverfire air prior to the introduction of the overfire air into theburnout zone.
 23. The method of claim 19, wherein the selective reducingagent, and overfire air, and flue gas form a mixture having atemperature briefly in a range of about 1600° F. to about 2000° F. andsaid decrease in the concentration occurs while the mixture temperatureis in said range, and the flue gas in the burnout zone has a temperatureabove 2000° F.
 24. The method of claim 19, wherein the overfire air isintroduced through at least two ports located at different levels ofsaid burnout zone, and said selective reducing agent is injected throughan upper port of said at least two ports.